The invention relates to the field of optical communication, and in particular to a technique of design for prescribed spectral characteristics for, and a means of realization of, optical coupled-resonator wavelength (channel add/drop) filters in which the resonators have finite loss, or of optical coupled-resonator wavelength-selective tap filters where only a fraction of the total power at a selected wavelength is to be extracted (e.g. for channel monitoring applications).
In wavelength-division-multiplexed (WDM) optical communication networks, the optical add/drop multiplexer (OADM) permits the extraction of one or more wavelengths from the signal spectrum of the bus fiber onto secondary optical paths, and the insertion of new optical signals into these same, now vacant, spectral slots. The channel add/drop filter (CADF) is the functional component of the OADM which performs the drop and add operations for one channel at a given (fixed or tunable) center wavelength. Generally filter characteristics with low loss, a flat-top and sharp rolloff drop-port response, and strong in-band extinction in the thru-port are desirable. Channel monitoring filters with flat-top drop-port responses are also desirable, that permit the extraction of only a fraction of the power in a particular wavelength channel without distortion of the extracted or remaining signals. Dense integration of CADFs and other filters, as well as complete OADMs, on a chip is desirable from both a technological and an economic point of view.
State-of-the-art integrated CADFs employ waveguide-coupled resonators (for their frequency selectivity), and are implemented in dielectric materials for the low absorption losses these provide at optical frequencies. Multiple-cavity coherent filter configurations such as series-coupled microrings and parallel-coherent microrings are known in the art and commonly used to achieve higher-order, flat-top band-pass/band-stop filter responses that fully extract a selected wavelength channel, for add/drop filter applications. In a series-coupled arrangement, each resonator is mutually coupled and for at least an output port (e.g. the drop port) an input signal that is outputted from the output port passes sequentially through each resonator For an idealized resonator system with lossless resonant cavities, couplings and waveguide propagation, models of these filter topologies permit design of the input-to-drop response function for maximally flat (Butterworth), equiripple (Chebyshev), etc. passbands in analogy to electronic filters. The resulting designs generally require synchronous cavities and symmetric coupling-coefficient distributions. In the absence of loss and reflection, the other three relevant amplitude response functions of the add-drop filter (i.e. input-to-thru, add-to-thru and add-to-drop) are fixed by power conservation and geometric symmetry.
In practice, the resonant cavities, their mutual couplings and waveguide propagation have finite losses due to radiation and absorption. Loss in cavities tends to degrade the CADF response by reducing the channel drop efficiency from 100%, rounding the flat-top drop-port passband, reducing the out-of-band rejection, degrading in-band rejection in the thru port and increasing by-pass losses seen by adjacent channels. Thus, considerable research effort has been invested to design and fabricate lower loss cavities to reduce the degradation due to loss.